During joining, e. g. welding or soldering, or when cutting workpieces by means of a laser beam, splashes or flue gases are usually produced which can contaminate the laser processing head, in particular the glass for decoupling the laser beam onto the workpieces. On the one hand, an increase in turbidity reduces the amount of laser power available for the machining process. On the other hand, adhering splashes can deflect and/or scatter the laser beam, whereby the laser energy absorbed by the splash can lead to damage (e. g. melting, craters and cracks) or even to breakage of the protective glass.
Protective-glass monitors are known from the prior art which detect the temperature of the protective glass and generate an error signal when a temperature limit value is exceeded. For example, EP 1 310 782 A1 discloses determining the temperature by measuring an electrical resistance. A method of monitoring a protective glass, in which a temperature sensor detects a temperature of the protective glass and compares it with a reference temperature value, is described in DE 101 13 518 A1. It is proposed in DE 10 2004 006 565 A1 to measure a change in the length of an optical element and subsequently to determine a temperature change from the change in length.
A disadvantage of these monitoring devices is their dependence on the ambient temperature, i. e. in cold environments the monitoring may react too late, since a higher degree of contamination is necessary to trigger the error signal in comparison to warm environments.
Systems based on scattered light measurement are also known. For example, WO 98/33059 A1 describes such a method, wherein the scattered light generated by contamination is detected by means of a photodetector.
However, contamination monitoring by means of scattered light measurement shows dependencies on the type of contamination (absorption or reflection of the measuring light) as well as on the position of the contamination on the protective glass. Also, it may be difficult to separate the scattered light from stray light resulting from the process, so monitoring may be unreliable.
DE 203 14 918 U1 shows a device for monitoring a protective glass of a laser optic for breakage and/or contamination, wherein at least one photodetector, which is arranged on the circumference of the end face of the protective lens and is connected to an evaluation device, is provided, to which at least one electromagnetic radiation source is assigned, the electromagnetic radiation of which is coupled via the end face of the protective glass. However, this device only detects disturbances occurring in the glass, and surface contaminants are hardly detectable.